1. Field of the Invention
This invention relates to a high-frequency device used in a communication unit, and more particularly to a resonator or filter or a high frequency device using the resonator or filter.
2. Description of the Related Art
A communication unit for communicating information by wire or radio is constructed by various devices such as an amplifier, mixer, and filter and includes various devices utilizing the resonance characteristic. For example, the filter has a plurality of resonance elements and has a function of permitting passage of signals only in a specified frequency band. The filter is required to have a small insertion loss and a characteristic for inhibiting passage of signals in a frequency band other than the desired frequency band, and in order to meet the requirement, it becomes necessary to use resonance elements having a large unloaded Q value.
As a method for realizing a resonance element having a large unloaded Q value, a method using a superconducting material as a metal conductor constructing the resonance element and using a material such as sapphire or MgO having an extremely small loss as a dielectric substrate is provided. However, in this case, since the unloaded Q value becomes 10,000 or more and an extremely sharp resonance characteristic can be attained, the desired characteristic cannot be attained if the resonance characteristic is not adjusted with high precision at the stage of design.
In order to overcome the above problem, a resonator and a filter having a function of adjusting the resonance frequency is proposed (Jpn. Pat. Appln. KOKAI Publication No. 1-190001, hereinafter referred to as Document 1). In this example, the resonance element is constructed by superconductors having two different values of critical magnetic field and the resonance frequency can be changed by adjusting the intensity of the magnetic field applied to the superconductors to deteriorate the superconducting characteristic thereof. However, with this method, it is necessary to use a magnetic generator for providing a strong magnetic field to change the characteristic of the superconductor, thereby making the size of the device large.
As another method, a method using a heater provided near the resonance element formed of a superconductor and deteriorating the superconducting characteristic by heat is known (Y. Nagai, D. F. Hebert and T. Van Duzer, Appl. Phys. Lett, Vol. 63, No. 6.9 August, 1993, p. 830, "Properties of superconductive bandpass filters with thermal switches", hereinafter referred to as Document 2). With this method, the device becomes simple in construction in comparison with the method using the magnetic field as is disclosed in the document 1, but this method has a problem that the operation speed becomes low since it is necessary to supply a current to the heater for heat generation. In order to enhance the operation speed, it is necessary to maintain the environment temperature of the filter at or near the critical temperature T.sub.c of the superconductor. However, it is extremely difficult to maintain such an environment temperature.
Further, in the methods disclosed in the documents 1 and 2, the low loss property of the superconductor is sacrificed by deteriorating the superconducting characteristic of the superconductor. For this reason, there occurs a problem that the low loss property and the adjustable range are in a trade-off relationship.
Further, as a still another method, a method for changing the resonance frequency by providing a gap in the central portion of the transmission line, disposing a dielectric material whose permittivity is changed by application of voltage on the gap portion and applying a voltage to the transmission line is provided (James A. Beall, Ronald H. Ono, David Galt and John C. Price, IEEE MTT-S Digest, 1993, P.1421, "TUNABLE HIGH TEMPERATURE SUPERCONDUCTOR MICROSTRIP RESONATORS", hereinafter referred to as Document 3). However, this method has a problem that only a resonator having a unloaded Q value of 1000 or less can be realized since a gap must be provided at the current maximum point in the central portion of the transmission line used as the resonance element.
Among the conventional techniques for changing the resonance frequency as described above, the technique for changing the resonance frequency by adjusting the intensity of the magnetic field applied to the resonance element including the superconductor and deteriorating the superconducting characteristic thereof requires a magnetic generator for providing a strong magnetic field to change the characteristic of the superconductor and the size of the device becomes large.
Further, in the technique for changing the resonance frequency by heating the superconductor of the resonance element and deteriorating the superconducting characteristic thereof, the operation speed is low and the low loss property of the superconductor is sacrificed.
Further, in the technique for changing the resonance frequency by disposing a dielectric material whose permittivity is changed by application of voltage on the gap portion provided at the center of the transmission line and applying a voltage to the dielectric substance via the transmission line, a resonator having a large unloaded Q value cannot be realized.